I would say they have to toe the line between makeing the most popular units of a given series easily available, getting their money out of a mold, and appeasing completionists.

Since Dai-Guard is, without question, the best known robot from his show, it makes sense to make him the wide-spread release and leave Kokubouga as a Web Exclusive for the few completionists that want him, at little cost to themselves since it used most of Dai-Guard's parts.

This is just a guess, of course. Kokubouga could be an all-new Mold like Mazinkaiser SKL was... SO ticked I couldn't get that thing...